Sound waves exist as variations of pressure in a medium, such as air. The pressure variation results from the vibration of an object, which causes the surrounding medium to vibrate. The vibrating medium (i.e., air) then causes the human eardrum to vibrate, which the brain interprets as sound.
A loudspeaker (or “speaker”) is an electroacoustic transducer that generates pressure variations in air in response to an electrical audio signal that it receives. The term “loudspeaker” may refer to individual transducers, known as audio “drivers,” or to complete speaker systems consisting of an enclosure, speaker binding posts, a cross-over, and one or more audio drivers.
The most common type of audio driver is a dynamic driver. It uses a lightweight conically configured diaphragm (i.e., a cone), which is connected to a rigid supporting basket via a flexible suspension called a spider. The spider constrains a coil of fine wire, known as a voice coil.
When a “music” signal, typically from an audio amplifier and in the form of an alternating current (“AC”), is applied to the voice coil, a magnetic field is generated due to the flow of electric current. The field fluctuates as a function of the applied AC music signal. This fluctuating electromagnetic field interacts with the magnetic field of a fixed permanent magnet. This interaction results in a mechanical force (i.e., the Lorentz force). The force causes the coil and, hence, the attached cone, to move back and forth. The air pressure in front of the cone increases and decreases as a consequence of this movement. Pressure waves, which the human ear/brain interprets as “sound,” are thereby generated in the air.
Drivers other than dynamic drivers are known and used for audio reproduction. Of particular relevance to this disclosure are planar drivers. Planar drivers are characterized by flat planar diaphragms (for moving air). One type of planar driver is the electrostatic transducer, the salient elements of which are depicted in FIG. 1. Electrostatic driver 100 includes three basic components: stators 102, diaphragm 106, and spacers (not shown).
Stators or grids 102 are electrically conductive metal sheets that are coated with an insulator. Stators 102 are perforated with holes 104. Diaphragm 106 is a very light weight plastic film, such as PET (commonly known as “Mylar”) having a thickness 2-20 μm. The film is treated with an electrically conductive material, such as graphite. Diaphragm 106 is stretched taut and is disposed in a gap between stators 102. To help stiffen the stators and to prevent the diaphragm from contacting a stator, electrically-insulating strips of material (not depicted) are placed widthwise at intervals along each stator's length.
In operation, diaphragm 106 is charged to a fixed positive voltage by a high-voltage power supply. This generates a strong electrostatic field around the diaphragm. Stators 102 are driven by the electrical audio signal, which is delivered thereto from an audio amplifier via a step-up transformer. Stators 102 are anti-phase (i.e., one is positively charged and the other is negatively charged). The “sign” and amount of charge is a function of the electrical “music” signal. As a result, a uniform electrostatic field proportional to the audio signal is produced between both stators. This causes a force to be exerted on electrically charged diaphragm 106, causing it to move towards one or the other of the stators, depending on the charge of either. The moving diaphragm generates pressure variations in the air on either side of the diaphragm. Holes 104 are required so that these air pressure variations are projected outward beyond the stators. Diaphragm 106 is driven by two stators, one on either side of it, because the force exerted on the diaphragm by a single stator would be unacceptably non-linear, resulting in distortion.
A second type of planar driver is the magnetic driver. This driver works similarly to an electrostatic driver, except that the diaphragm is urged to movement due to a magnetic interaction rather than an electrostatic interaction. There are two types of magnetic drivers: ribbons and planar-magnetics.
FIG. 2 depicts ribbon driver 200. The ribbon driver includes two spaced-apart magnets 208A and 208B and “ribbon” 206.
The north pole of magnet 208A opposes the south pole of magnet 208B. Disposed between the two magnets in a side-by-side configuration is a flexible sheet of electrically conductive material known as a ribbon. Ribbon 206 is typically aluminum. The ribbon is very thin—essentially a foil—and it is pleated. Ribbon 206 is clamped at each end 210. There is slack in the ribbon such that it is under very little tension. The pleating serves at least two functions. It provides freedom of motion for the metal foil; that is, it serves as a suspension. It also provides a force-distribution function. More particularly, the Lorentz force (which arises as a consequence of magnetic interactions) acts on the edges of the ribbon as a function of frequency. The pleating helps to distribute the force so that it acts more evenly across the ribbon.
In operation, ribbon 206 receives the electrical (music) signal from an audio amplifier at contacts/terminals 212. Since the ribbon is conducting an electrical current, it generates a magnetic field. The magnetic field generated by ribbon 206, which varies as a function of the music signal, interacts with the non-varying magnetic field of permanent magnets 208A/B. Ribbon 206 is moved in either the plus or minus direction, perpendicular to the magnetic field between 208A/B, depending on which direction the alternating current of the amplifier's output is flowing.
The main advantage of a ribbon driver is that the ribbon has very little mass. Therefore, the ribbon can accelerate very quickly, providing excellent high-frequency response. But because it is so thin and light, the ribbon driver is exceedingly fragile and is very sensitive to outside pressure changes in the air.
Furthermore, ribbon drivers are generally limited to use as high frequency drivers since it is difficult to build a true ribbon driver large enough to handle lower frequencies. In particular, the pleated shape is easily damaged by its own weight if it's too large. Also, it can be damaged by excessive excursion, which can stretch and flatten the pleats. When this happens, the ribbon sags in the gap between the magnets and performs poorly. As a consequence, these drivers tend to be small and limited to high frequencies, wherein excursions are minimal and the ribbon can readily support its own weight.
Also, a true ribbon speaker presents a problematically low impedance to the power amplifier that drives it. The small ribbon presents little resistance to current flow, which, if not addressed, would behave like a short circuit to the amplifier. Even if the amplifier itself is not damaged, or doesn't shut down for protection, few amplifiers perform well when presented with such a low impedance load (about 0.1Ω). Most ribbons drivers incorporate a step-down transformer to mitigate this problem. Unfortunately, as with electrostatic speakers, the transformer itself can limit the ultimate performance of the ribbon driver and adds to its cost.
Unlike the ribbon driver in which the diaphragm is in a side-by-side arrangement with respect to the magnets, in a planar magnetic driver, the magnets are located in front of and, in some cases, also in back of the diaphragm. A thin flexible plastic, such as Mylar, typically serves as the diaphragm.
In some planar magnetics, a thin, flat, conductive foil is glued onto the diaphragm. In some other planar magnetic drivers, metal wire, rather than conductive foil, is glued to diaphragm. The electrically conductive element (i.e., foil or wire) conducts the amplifier's output (music) signal and creates an electromagnetic field that varies with the music signal. The varying electromagnetic field interacts with nearby permanent magnets giving rise to the Lorentz force that causes the diaphragm to move towards or away from the magnets. The planar magnetic driver can operate satisfactorily at much lower frequencies than a ribbon driver.
A cutaway view of planar magnetic driver 300 with wire is depicted in FIG. 3A. Driver 300 includes stators 302, diaphragm 306, permanent magnets 308A/B, and electrically conductive trace(s) or wire(s) 314.
As depicted in FIG. 3A, plural magnets 308A are disposed on one stator 302 and plural magnets 308B are disposed on the opposing stator. The magnets are arranged so that, along a stator, the north and south poles of adjacent magnets alternate. Magnetic field lines exit north poles and enter south poles. Stators 302, which are steel, close the magnetic circuits and secure magnets 308A/B in a proper orientation. The component of the vector of the magnetic field B that is useful (for generating movement) lies in the plane of diaphragm 306.
As in an electrostatic driver, diaphragm 306 is disposed between the two stators 302. More precisely, in driver 300, diaphragm 306 is disposed in a gap between opposing magnets 308A/B on the stators. The diaphragm is typically formed from Mylar or a material having characteristics similar thereto. Bonded to diaphragm 306 is one or more lengths of wire 314 arranged as one or more elongated coils. As depicted in FIG. 3B, the “coil” is stretched out along the diaphragm such the wire or electrically-conductive trace (hereinafter simply “wire”) 314 runs lengthwise, in parallel to bar magnets 308A/B.
In operation, an amplified electrical (music) signal is brought to the speaker's binding posts 312, which are electrically coupled to wire(s) 314. The current (I) flowing through the wire(s) generates a magnetic field (B) that varies as a function of the applied electrical signal. This fluctuating magnetic field interacts with the magnetic fields of the permanent magnets. A (Lorentz) force (F) results from the interaction; the force varies in magnitude as a function of the amplitude of the music signal and varies in direction as a consequence of the direction in which the current flows through wire(s) 314. The directional relationship between current (I), magnetic field (B), and force (F), as given by Flemings “left hand rule,” is depicted in FIG. 3A.
The resulting (Lorentz) force F is perpendicular to diaphragm 306, in one direction or the other. This force causes the wire(s) 314 and diaphragm 306 to move toward one set of magnets or the other. When the current changes direction, the direction of force changes 180 degrees. Wires 314 are arranged on the surface of diaphragm 306 so that the resulting force moves all of the wires (and hence diaphragm 306) in the same direction.
Because wires 314 are strongly bonded to diaphragm 306, and because wires 314 cover a major portion (about 80 percent or more) of the diaphragm's surface, the diaphragm moves back and forth like a piston. This movement, which is ultimately a consequence of the changing electrical (music) signal, vibrates the surrounding air, thereby creating a pressure (sound) wave. The pressure wave passes through openings 304 in both stators 302.
Planar magnetic drivers are relatively inefficient, with a low force-per-square-inch acting on the diaphragm. The active magnetic force is a line force, which can result in irregular movement of the diaphragm. Efficiency can be improved through the use of stronger magnets, but this exacerbates any tendency toward irregular diaphragm movement due to the nature of the line force, among other things. The driver performs with reasonable levels of distortion, but the frequency spectrum can have some sharp peaks in sound pressure level (“SPL”).
There is, as such, room for improvement in audio-driver technology. In particular, it would be advantageous to have a planar driver that has a broader frequency range of operation than existing planar drivers and that is more accurate with very low distortion over the full operating frequency range.